The surface acidic properties of calcined and reduced CoAPO-18 catalysts are reported. FTIR spectroscopy has revealed that the reduced template-free catalyst has bands at 3571 and 905 cm(-1) which have been assigned, respectively, to stretching and in-plane bending vibrations of hydroxyl groups bridging Co(II) and P [nu(OH) and delta(OH)]. It is proved that Co(II) substitutes for Al in the lattice of the ALPO-18 structure. Bridged hydroxyls are the locus of Bronsted acidity, the strength of which has been tested by adsorption of H2O, It is revealed that some bands produced by interaction between bridged hydroxyls and water are sensitive to the acidic strength of the Bronsted centers; this strength is found to exceed that of the structurally similar HSAPO-34. Calcined CoAPO-18 has Co(III) centers and the Bronsted acidity disappears, and it is deduced that redox Co(II)/Co(III) couples are intimately associated with the presence of Bronsted acid sites. Adsorption of H2O has revealed that Lewis acid centers are also present on both calcined and reduced CoAPO-18 catalysts. Both the spectroscopic features and the acidity of OH and OD groups are sensitive to the presence of Co in the structure of aluminophosphate molecular sieves, and an analysis of the OH and OD vibrations (both stretching and bending) in SAPO-34 and CoAPO-18 is presented.